1. Field of the Invention
The invention relates to a method and a printing unit for influencing lateral web spreading in rotary presses.
2. Description of the Related Art
When two cylinders of a rotary press are rolling on one another, line loads or else individual bending forces which cause the cylinders to deflect between their two bearings on the machine frame act on the cylinders along the common line of contact perpendicular to the axes of rotation of the cylinders. This has a disadvantageous effect on the paper-web run and the printing result. In slim cylinders having small diameters and great web widths, web-run problems in the form of wave or fold formation occur in the rubber-rubber nip on account of non-constant contact conditions across the width. In particular, the abovementioned web-run problems occur when the paper web is moved here with an irregular velocity profile because of the previously described positional change or non-constant contact conditions across the width, the center of the paper web being moved more quickly than the outer side of the web, which leads to fold formation. However, the web-transport behavior in web-fed offset presses is influenced critically by the conveying characteristics of the rubber blankets. In particular in printing presses having a slim cylinder geometry, fold formation can occur across the web width, which folds have a negative influence on the lateral-register quality.
To eliminate this problem, rubber-blanket sleeves have already been configured with a variable surface geometry across the web width, for example in U.S. Pat. No. 6,283,027, that is to say a thickness profile varied by the circumferential surface assuming a shape in the axial direction of the cylinder which reduces deflection of the blanket cylinder.